Electronic Article Surveillance (EAS) systems are widely used to detect the presence of small electronic labels placed on or in an article or carried by a person of interest, and can, for example, be used in retail or library environments to deter theft or any other unauthorized removal of articles. In particular, the EAS labels are attached to articles designated to either be purchased or borrowed, and when active, will trigger an alarm if carried without authorization through interrogation zones typically located at the store or library exits.
There are many applications when it is also desired to know more information than just whether a tagged object is present. For example, very often users also want to know which tagged article is present. In this connection, information-carrying tags are widely used both for the identification of products and security purposes, as well as product authentication. These information tags are able to carry a sufficient number of bits regarding the characteristics of articles to provide useful information, such as the product's name, date of manufacture, price, and whether the product, article or person has properly passed through a check-out counter or kiosk, etc. As used herein, the terms “information carriers”, “marks”, “markers”, “labels” and “tags” are used interchangeably and refer to the devices used to carry multi-bit data therein.
The most popular example of the information-carrying multi-bit tags is a printed optical barcode. While inexpensive and effective, the optical bar code system, however, has certain limitations, for example, the “words” utilized in coding the bar code, have a relatively long length due to their binary nature. In fact, these words consist of a plurality of two ordinal symbols (“0” or “1”) allocated for each bit and represented by the presence or absence of the barcode's lines.
Various examples of multi-bit markers are known in the art, which can employ more than two symbols for encoding information therein. One known kind of such markers utilizes selected combinations of multiple different diffraction gratings as a means for forming indicia, e.g., marking price, inventory number and the like.
For example, U.S. Pat. No. 4,011,435 to Phelps et al. describes an optical marking system which utilizes tags that are formed from multiple diffraction gratings. Each of the gratings differs from the others in orientation and number of lines (spatial frequency), in accordance with the coded information desired on the tags. The concept of coding is based on the diffraction patterns obtained when combinations of gratings with pre-selected orientations and number of lines are illuminated with coherent light. A specific coding is thus represented by a specific arrangement of images in the diffraction part; the images in turn are associated with the angular orientations and spatial frequencies of the gratings. A unique group of images in the diffraction pattern may represent a unique group of information bits.
U.S. Pat No. 4,034,211 to Horst et al. describes, inter alia, an optical identification system using a line of several different, quite discrete and distinct optical gratings which are imbedded in a predetermined or within the construction of a credit card. Optical gratings (each being characteristic of a code value) consist of a number of parallel straight lines at a given spacing across a surface. The lines may exist in the form of opaque printed lines, transparent slots, reflective bars or lines cut into the surface of an object. Optical gratings can be illuminated by a monocromatic beam of light to cause diffraction. The diffracted rays can be detected by a photodetector placed at a precisely determined location. The presence or absence of a particular grating will therefore produce a digital-type electrical signal from the associated photodetector.
U.S. Pat. No. 5,291,027 to Kita et al. describes an identification mark having a plurality of mark elements. At least two different diffraction gratings (having different directions and/or different spatial frequencies) can be drawn in each mark element of the identification mark so that a pattern having a binary character is formed. Each mark element is divided into a large number of first and second square fine regions, which are alternatively arranged. The first grating is formed in only the first fine regions, and the second gang is formed in only the second fine regions. When desired, the number of types of diffraction grating patterns which one mark element can produce can be increased. Therefore, the information volume, which can be carried by the identification mark can also be increased.
U.S. Pat. No. 5,811,775 to Lee describes an optical data element that includes a plurality of diffraction zones wherein each zone contains a multiplicity of diffraction gratings. The diffraction gratings can define diffraction grating pixels which for the respective diffraction zones are chosen from a palette of pixel grating functions. A simple palette can comprise a range of straight line grating functions having varying grayscale values. For example, the palette comprising 16 different functions and corresponding grayscale values can define 16 element palette from palette element K to palette element Z, uniformly varying in steps from near white to substantially black. Each of these grating pixels, when illuminated, will generate a first order diffracted beam whose divergence angle increases from palette element Z to palette element K. If a detector is placed at a given distance in front of the grating to detect this first order beam, the intensity recorded at the detector will likewise decrease in a stepped scale from palette element Z to palette element K.
Despite the extensive prior art existing in the area of multi-bit information carriers based on barcodes and optical diffraction gratings, there are certain disadvantages associated with the utilization of such devices. One of the main drawbacks of the techniques based on the use of bar codes and optical diffraction gratings is related to the fact that the bar codes and optical diffraction gratings must be visible, which limits the locations in which they may be placed. Thus, bar codes and optical diffraction gratings can easily be obscured, either accidentally or intentionally. The visible barcodes and optical diffraction gratings can be easily duplicated. Therefore, as soon as a counterfeiter learns the method of duplication, the barcodes and optical diffraction gratings become worthless for authentication purposes. Moreover, because barcodes and optical diffraction gratings are exposed to ambient medium, the barcodes and optical diffraction gratings are susceptible to damage that can result in detection failures. Usually, the ranges at which detectors can sense bar codes and optic diffraction patterns are also comparatively small. The bar codes and optical diffraction gratings should also be appropriately positioned for radiation and detection. Therefore, the barcodes and optical gratings cannot be placed beyond the line of sight of the detecting device. Last, but not least, the disadvantage is also associated with the fact that multiple items utilizing the barcodes and optical diffraction gratings must be scanned (read) one at a time. These constraints of bar code systems and optical diffraction gratings make them undesirable or inefficient for some applications.
Another technique of electronic item identification is based on the use of a Radio Frequency (RF). RF tags have been developed and are disclosed, for example, in the following publications: U.S. Pat. Nos. 5,574,470 to de Vall; 5,625,341 to Giles et al.; 5,682,143 to Brady et al; 5,995,006 to Walsh; 6,100,804 to Brady et al.; 6,346,884 to Uozumi et al; 6,424,263 to Lee et al; 6,441,740 to Brady et al. Such information carrying tags are responsive to a coded RF signal received from a base station.
Generally, RF tags can be active (utilizing an internal energy source incorporated with the tag) or passive, that functions using the energy of an external interrogation signal and are dependant on energy supplied from a tag reader or an external device. The RF tag typically includes an antenna attached to a resonance circuit which is energized by the received interrogation signal and which, when energized, excites the antenna to transmit a response radio frequency signal. The antennas used in an RF ID tag are generally constituted by loops of wire or metal, etched or plated on the tag surface.
The particulars and advantageous features of active and passive RF tags are known.
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RF tags may have various code or memory configurations. The simplest example is the single-code tag typically utilized in EAS systems. These tags emit a single response when activated by a reader. The response is a simple yes or no, indicating whether or not the tag is present or activated. Other tags respond with a code including a preset serial number. It is also known to provide a plurality of resonant circuits on an RF ID tag, each resonant circuit configured with the tag to output a response signal at a predetermined frequency in response to a query signal. The number of possible different responses is determined by the number of individual circuits and/or the ability to time sequence the responses from the circuit.
For example, U.S. Pat. No. 6,104,311 to Lastinger describes an RF identification tag comprising a substrate, an input mechanism disposed on the substrate and configured to receive a query signal, and an output mechanism disposed on the substrate. The tag includes a response circuitry disposed on the substrate in operative communication with the input/output mechanism which can be one or more antennas configured to receive and transmit signals at a predetermined frequency. The response circuitry includes one or more code circuits, each configured to output a unique response code. A connection between any given code circuit and a given antenna determines the response code and frequency at which that response code will be generated and output in the response signal. The selective connection of the code circuits and the antennas determines the response code/frequency combinations that comprise the response signal.
Another tag which uses radio frequency waves transmitted from a scanning device in order to identify an item to which the tag is attached is described in U.S. Pat. No. 6,304,169 to Blama. The tag includes a first insulating layer having a top surface and a bottom surface, and resonant circuits formed on the first insulating layer. Each of the resonant circuits are formed on either the top surface or the bottom surface of the first insulating layer and have a resonant frequency associated therewith. Each of the resonant circuits includes a resonance circuit (capacitance and inductance elements). The tag is associated with a binary number established by a pattern of ones and zeros depending on each circuit resonance or non-resonance, respectively.
The developments in the RF tag technology are aimed at creating the so-called chipless RFID tags (utilizing a printing technique rather than the integrated technology), such as is known.
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